The present invention relates to cellular communication systems, and more particularly to techniques and apparatuses for allocating synchronization sequences (“codes”) to cells in a cellular communication system.
To facilitate the following discussion, terminology and network configurations that comply with the Third Generation Long Term Evolution (LTE) standard are used herein because these are known and will be readily understandable to the person of ordinary skill in the art. However, the use of this terminology and these configurations is done solely for the purpose of example rather than limitation. The various inventive aspects to be described in this document are equally applicable in many different mobile communications systems complying with different standards.
In the forthcoming evolution of the mobile cellular standards like the Global System for Mobile Communication (GSM) and Wideband Code Division Multiple Access (WCDMA), new transmission techniques like Orthogonal Frequency Division Multiplexing (OFDM) are likely to occur. Furthermore, in order to have a smooth migration from the existing cellular systems to the new high-capacity high-data rate system in existing radio spectrum, a new system has to be able to utilize a bandwidth of varying size. The LTE system has been developed for this purpose. It is a new flexible cellular system that can be seen as an evolution of the 3G WCDMA standard. This system will use OFDM as the multiple access technique (called OFDMA) in the downlink and will be able to operate on bandwidths ranging from 1.25 MHz to 20 MHz. Furthermore, data rates up to and exceeding 100 Mb/s will be supported for the largest bandwidth. However, it is expected that LTE will be used not only for high rate services, but also for low rate services like voice. Since LTE is designed for Transmission Control Protocol/Internet Protocol (TCP/IP), Voice over IP (VoIP) will likely be the service that carries speech.
FIG. 1 illustrates a mobile communication service area 101, such as an LTE system service area, that comprises a number of cells 103. User Equipment (UE) (e.g., the UE 105) located in a cell is served by an antenna in that cell. The antenna is coupled to a node in the communication system so that communication data can be routed between the UE and other equipment through the communication system.
A simplified cell planning diagram is depicted in FIG. 2. A core network (not shown) is connected to one or more evolved UTRAN Node Bs (eNodeB) (201-1, 201-2) (generally referred to by means of reference numeral 201). Each eNodeB 201 is capable of communicating with every other eNodeB 201 in the same network. As can be seen in FIG. 1, one eNodeB 201 connects to one or more antennas, 203-1, 203-2, . . . , 203-M (generally referred to by the reference numeral 203). The eNodeB 201 is a logical node handling the transmission and reception of a set of cells. Logically, the antennas of the cells belong to the eNodeB but they are not necessarily located at the same antenna site. Thus, one eNodeB 201 can be responsible for one or more cells. It is the ability of serving cells not transmitted from the same antenna site that makes a NodeB different compared to what in other types of systems are called a “Base Transceiver Station (BTS)”, “Base Station (BS)”, or “Radio Base Station (RBS)”. However, in this specification the term “base station” is used as a generic term, rather than a system-specific term, to further emphasize that the invention is not limited to applications in only the specific exemplary systems.
The LTE physical layer downlink transmission is based on OFDM. The basic LTE downlink physical resource can thus be seen as a time-frequency grid as illustrated in FIG. 3, in which each so-called “resource element” corresponds to one OFDM subcarrier during one OFDM symbol interval.
As illustrated in FIG. 4, the downlink subcarriers in the frequency domain are grouped into resource blocks, where each resource block consists of twelve consecutive subcarriers for a duration of one 0.5 ms slot (7 OFDM symbols when normal cyclic prefixes are used (as illustrated) or 6 OFDM symbols when extended cyclic prefixes are used), corresponding to a nominal resource-block bandwidth of 180 kHz.
The total number of downlink subcarriers, including a DC-subcarrier, thus equals Nc=12·NRB+1 where NRB is the maximum number of resource blocks that can be formed from the 12·NRB usable subcarriers. The LTE physical-layer specification actually allows for a downlink carrier to consist of any number of resource blocks, ranging from NRB-min=6 and upwards, corresponding to a nominal transmission bandwidth ranging from around 1 MHz up to well beyond 20 MHz. This allows for a very high degree of LTE bandwidth/spectrum flexibility, at least from a physical-layer-specification point-of-view.
FIGS. 5a and 5b illustrate the time-domain structure for LTE downlink transmission. Each 1 ms sub-frame 500 consists of two slots of length TSlot=0.5 ms (=15360·TS, wherein each slot comprises 15,360 time units, TS). Each slot then consists of a number of OFDM symbols.
A subcarrier spacing Δf=15 kHz corresponds to a useful symbol time Tu=1/Δf≈66.7 μs (2048·TS). The overall OFDM symbol time is then the sum of the useful symbol time and the cyclic prefix length TCP. Two cyclic prefix lengths are defined. FIG. 5a illustrates a normal cyclic prefix length, which allows seven OFDM symbols per slot to be communicated. The length of a normal cyclic prefix, TCP, is 160·TS≈5.1 μs for the first OFDM symbol of the slot, and 144·TS≈4.7 μs for the remaining OFDM symbols.
FIG. 5b illustrates an extended cyclic prefix, which because of its longer size, allows only six OFDM symbols per slot to be communicated. The length of an extended cyclic prefix, TCP-e, is 512·TS≈16.7 μs.
It will be observed that, in the case of the normal cyclic prefix, the cyclic prefix length for the first OFDM symbol of a slot is somewhat larger than those for the remaining OFDM symbols. The reason for this is simply to fill out the entire 0.5 ms slot, as the number of time units per slot, TS, (15360) is not evenly divisible by seven.
When the downlink time-domain structure of a resource block is taken into account (i.e., the use of 12 subcarriers during a 0.5 ms slot), it will be seen that each resource block consists of 12.7=84 resource elements for the case of normal cyclic prefix (illustrated in FIG. 4), and 12·6=72 resource elements for the case of the extended cyclic prefix (not shown).
Another important aspect of a terminal's operation is mobility, an aspect of which is cell search. Cell search is the procedure by which the terminal finds a cell to which it can potentially connect. As part of the cell search procedure, the terminal obtains the identity of the cell and estimates the frame timing of the identified cell. The cell search procedure also provides estimates of parameters essential for reception of system information on the broadcast channel, containing the remaining parameters required for accessing the system.
To avoid complicated cell planning, the number of physical layer cell identities should be sufficiently large. For example, systems in accordance with the LTE standards support 504 different cell identities. These 504 different cell identities are divided into 168 groups of three identities each.
In order to reduce the cell-search complexity, cell search for LTE is typically done in several steps that make up a process that is similar to the three-step cell-search procedure of WCDMA. To assist the terminal in this procedure, LTE provides a primary synchronization signal and a secondary synchronization signal on the downlink. This is illustrated in FIG. 6, which illustrates the structure of the radio interface of an LTE system. The physical layer of an LTE system includes a generic radio frame 600 having a duration of 10 ms. FIG. 6 illustrates one such frame 600 for an LTE Frequency Division Duplex (FDD) system. Each frame has 20 slots (numbered 0 through 19), each slot having a duration of 0.5 ms which normally consists of seven OFDM symbols. A sub-frame is made up of two adjacent slots, and therefore has a duration of 1 ms, normally consisting of 14 OFDM symbols. The primary and secondary synchronization signals are specific sequences, inserted into the last two OFDM symbols in the first slot of each of subframes 0 and 5. In addition to the synchronization signals, part of the operation of the cell search procedure also exploits reference signals that are transmitted at known locations in the transmitted signal.
In the first step of the cell-search procedure, the mobile terminal uses the primary synchronization signal to find the timing of the 5 ms slots. Note that the primary synchronization signal is transmitted twice in each frame. One reason for this is to simplify handover of a call from, for example, a GSM system, to an LTE system. However, transmitting the primary synchronization signal twice per frame creates an ambiguity in that it is not possible to know whether the detected Primary Synchronization Signal is associated with slot #0 or slot #5 (see FIG. 6). Accordingly, at this point of the cell-search procedure, there is a 5 ms ambiguity regarding the frame timing.
In many cases, the timing in multiple cells is synchronized such that the frame start in neighboring cells coincides in time. One reason for this is to enable MBSFN operation. However, synchronous operation of neighboring cells also results in the transmission of the primary synchronization signals in the different cells occurring at the same time. Channel estimation based on the primary synchronization signal will therefore reflect the composite channel from all such cells if the same primary synchronization signal is used in those cells. For coherent demodulation of the second synchronization signal, which is different in different cells, an estimate of the channel from the cell of interest is required, not an estimate of the composite channel from all cells. Therefore, LTE systems support multiple (presently three) sequences for the primary synchronization signals. To enable coherent reception of a particular cell's signals in a deployment with time-synchronized cells, neighboring cells are permitted to use different primary synchronization sequences to alleviate the channel estimation problem described above. If there is a one-to-one mapping between the primary synchronization signal used in a cell and the identity within a cell identity group, the identity within the cell identity group can also be determined in the first step.
In the next step, the terminal detects the cell identity group and determines the frame timing. This is done by observing pairs of slots in which the secondary synchronization signal is transmitted. To distinguish between secondary synchronization signals located in subframe #0 and subframe #5, the secondary synchronization signals are constructed in the form (S1, S2). If (S1, S2) is an allowable pair of sequences, where S1 and S2 represent the secondary synchronization signal in subframes #0 and #5, respectively, the reverse pair (S2, S1) is not a valid sequence pair. By exploiting this property, the terminal can resolve the 5 ms timing ambiguity that resulted from the first step in the cell search procedure, and determine the frame timing. Furthermore, as each combination (S1, S2) represents a particular one of the cell groups, the cell group identity is also obtained from the second cell search step. The identity of the cell than can be used to determine the reference (or pilot) signal sequence and its allocation in the time-frequency grid.
The synchronization signals occupy 62 resource elements in the center of the allocated bandwidth. Five resource elements on either side of the 62 resource elements are set to zero, making a total of 72 resource elements in which the synchronization signals can be found during subframes #0 and #5 as described above. To distinguish between the secondary synchronization signal S1 and the secondary synchronization signal S2, each is created as a function of a pair of sequences {tilde over (S)}i, {tilde over (S)}j. That is, S1=ƒ({tilde over (S)}1, {tilde over (S)}2) and S2=ƒ2({tilde over (S)}1, {tilde over (S)}2), as illustrated in FIG. 7a. Each of the sequences: {tilde over (S)}i, {tilde over (S)}j, is one of 31 different M-sequences, which is essentially a certain pn-sequence.
In LTE, the function for deriving S1 and S2 is implemented in the frequency domain by transmitting the sequences {tilde over (S)}i, and {tilde over (S)}j simultaneously by means of interleaving. For example, given two sets of frequencies that are interleaved with one another, transmission of the symbol S1 can be performed by transmitting the sequence {tilde over (S)}i, in a “lower” one of the sets of interleaved frequencies and transmitting the sequence {tilde over (S)}j, in a “higher” one of the sets of frequencies. (Here, the words “higher” and “lower” do not refer to the sets of frequencies as a single contiguous group, but rather to pairs of resource elements associated with the interleaved frequencies, so that one resource element associated with {tilde over (S)}i, is on a lower frequency than the neighboring resource element associated with {tilde over (S)}j.) Transmission of the symbol S2 is the opposite, with the sequence {tilde over (S)}j being transmitted in a lower one of the sets of frequencies and the sequence {tilde over (S)}i being transmitted in a higher one of the sets of frequencies. This arrangement is illustrated in FIG. 7b. (To simplify the diagram, the unused DC carrier is not shown in FIG. 7b.)
A UE preferably includes a look-up table that associates each sequence pair and ordering with a cell group identifier and frame timing information (i.e., whether the ordering of the sequence pair indicates sub-frame 0 or sub-frame 5), so that the UE can easily identify the cell group and frame timing.
Once the cell search procedure is complete, the terminal receives the system information to obtain the remaining parameters (e.g., the transmission bandwidth used in the cell) necessary to communicate with this cell. This broadcast information is transmitted on the BCH transport channel.
The secondary synchronization signals can also be used for purposes other than identifying frame timing cell groups. For example, these signals are useful for assisting positioning services to determine a geographical position of a mobile terminal. The possibility of determining the position of a mobile device has enabled application developers and wireless network operators to provide location based, and location aware, services. Examples of such services include guiding systems, shopping assistance, friend finder, presence services, community and communication services and other information services giving the mobile user information about their surroundings.
In addition to the commercial services, governments in several countries require network operators to be able to determine the position of an emergency call. For instance, governmental regulations in the USA (FCC E911) require that it must be possible to determine the position of a certain percentage of all emergency calls. The requirements make no difference between indoor and outdoor environments.
The position estimation can be achieved by means of positioning systems, (e.g. Global Positioning System (GPS)-based methods like Assisted-GPS (A-GPS)). Position estimation can also be performed using the wireless network itself. Methods using the wireless network can be grouped into two main types. The first type comprises methods that are based on the radio cell to which a mobile terminal is attached (e.g. by using Cell-ID or a combination of cell-ID and Timing Advance (TA) information). Here the primary and secondary synchronization signals become relevant. Since each neighboring cell has a unique primary and secondary synchronization signal combination, these signals can be used as a kind of pilot signal to determine the cell timing for a respective cell.
WO 08/144,746 describes an approach for allocating primary synchronization signal and secondary synchronization signal sequences to cells in an LTE system. However, nothing in the disclosed technique is particularly helpful with respect to the quality (e.g., accuracy) of positioning system performance.
It is therefore desired to provide methods and apparatuses that facilitate the use of secondary synchronization signals (and their equivalent in other systems) to enhance the performance of services such as those utilizing positioning systems.